The present invention relates generally to gas generating compositions and specifically to a gas generating composition with improved ballistic properties suitable for use in automatically inflating inflatable structures such as vehicle airbags and aircraft escape chutes.
Gas generating compositions have long been used for a multitude of purposes. The requirement for vehicular, especially automotive, airbags in passive restraint systems designed to protect drivers and passengers in the event of a collision has produced a substantial amount of research for the ideal gas generating composition for this purpose. The ideal gas generating composition should be a thermally stable, cool burning, noncorrosive composition that generates a large volume of substantially ash or solids-free clean, nontoxic gas. The ideal gas generating composition should also be storage stable so that it ignites effectively and burns efficiently when needed. While the prior art has proposed gas generating compositions that approach this ideal, it has not yet been achieved for automatically inflated structures such as vehicle airbags.
The current state of the art gas generating compositions, referred to in the gas generator technology as xe2x80x9cpropellantsxe2x80x9d, typically include an ammonium nitrate oxidizer, either combined with a rubbery binder or in a pressed charge to form a pellet, which is stored until ignited to inflate the airbag or other structure. Various chemical additives, for example highly oxygenated fuels such as guanidine nitrate, aminoguanidine nitrate and oxamide are combined with the ammonium nitrate to aid ignition, modify burning rates, promote smooth burning and produce acceptably low flame temperatures. Combustion catalysts may be included in the composition to increase burning rate, promote ignition and low pressure combustion. However, the metallic additives often used in combustion catalysts produce solids in the effluent gas that may interfere with the gas toxicity, exhaust particulates, or inflation of the airbag or other inflatable structure.
Ammonium nitrate is the most commonly used oxidizer in these types of gas generator compositions. It is readily available, safe to handle, and inexpensive. Moreover, ammonium nitrate burns at low flame temperatures and burning rates to produce a nontoxic, noncorrosive exhaust. Primary disadvantages of using ammonium nitrate as the oxidizer in a gas generator composition are inherently low burning rates, higher pressure exponents, poor combustion at low pressures, and its tendency to undergo phase changes during temperature variations, which causes cracks and voids in the pellet. Cracked pellets are not likely to yield a reliable gas generator when needed. Crack formation can be minimized by employing a binder that is sufficiently strong and flexible to hold the composition together. Pellets formed without a binder additive will crack unless phase change additives are used and/or specific additional components or processing steps are employed.
U.S. Pat. No. 5,545,272 to Poole et al. is illustrative of an ammonium nitrate based gas generating composition for an automobile airbag. The mechanical mixture of ammonium nitrate, nitroguanidine, and a potassium salt described by Poole et al. suffers from some of the drawbacks discussed above, however. This type of composition is subject to the aforementioned ammonium nitrate phase changes due to temperature cycling. Since the composition does not include a binder or phase change modifying component and is not produced to modulate ammonium nitrate phase changes, cracks and voids in the gas generating pellet are a likely result.
U.S. Pat. No. 5,551,725 to Ludwig discloses an inflator composition for a vehicle airbag that includes an oxidizer, such as ammonium nitrate, and a fuel, which may be a nitro-organic, such as guanidine nitrate. The Ludwig composition, like the Poole et al. composition, would not avoid potentially detrimental ammonium nitrate phase changes.
In the foregoing gas generating compositions, as well as in other available gas generators, the burning rates tend to be low and the pressure exponent values tend to be high so that they are not as efficient as desired. These ballistic properties pose challenges in the design of a vehicle airbag unit. Low burning rates lead to high operating pressures and/or thin web designs. High exponents at low pressures lead to poor and variable combustion and unburned residues. Moreover, under these conditions, the thin web designs typically used for the gas generator charges weaken and become friable and are susceptible to vibrational damage so that the storage stability of the gas generator is compromised.
A need exists, therefore, for a thermally stable, storage stable gas generating composition characterized by both an increased burning rate and a lower pressure exponent than heretofore achieved that produces a clean, nontoxic, substantially ash- and solids-free gas at optimum combustion efficiency and operating pressure.
It is a primary object of the invention therefore, to provide a thermally stable, storage stable gas generating composition characterized by both an increased burning rate and a lower pressure exponent than heretofore achieved that avoids the disadvantages of the prior art and produces a clean, nontoxic, substantially ash- and solids-free gas at optimum combustion efficiency and operating pressure.
It is another object of the present invention to provide a gas generating composition with improved ballistic properties that is ideally suited for effectively and efficiently inflating a vehicle airbag.
It is a further object of the present invention to provide a gas generating composition characterized by an optimum burning rate and an optimum pressure exponent value for automatically inflating inflatable structures such as vehicle airbags, aircraft escape chutes and the like.
It is yet another object of the present invention to provide a gas generating composition that exhibits desirable thermal aging, thermal cycling and pellet strength characteristics.
It is yet a further object of the present invention to provide a catalyst for a gas generating composition with improved ballistic properties that includes a solid solution or eutectic mixture of ammonium nitrate, at least one highly oxygenated fuel, preferably guanidine nitrate (GN) and/or aminoguanidine nitrate (AGN), and an optional phase stabilizer.
It is still another object of the present invention to provide a method for simultaneously increasing the burning rate and lowering the pressure exponent of gas generating compositions containing a eutectic mixture or solid solution of ammonium nitrate, GN and/or AGN, a binder and an optional phase stabilizer without adversely affecting other properties by adding a selected catalyst to the gas generating composition.
The foregoing objects are achieved by providing a gas generating composition with improved ballistic properties comprising a solid solution or eutectic mixture of ammonium nitrate, GN and/or AGN, a phase stabilizer, a binder, and an effective amount of a catalyst selected from copper or copper compounds. A first preferred improved gas generating composition in accordance with the present invention comprises a solid solution or eutectic mixture of ammonium nitrate, guanidine nitrate and/or aminoguanidine nitrate, potassium or cesium nitrate, polyvinyl alcohol, and copper phthalocyanine. A second preferred improved gas generating compound comprises a eutectic mixture or solid solution of ammonium nitrate, guanidine nitrate and/or aminoguanidine nitrate, potassium perchlorate, polyvinyl alcohol and copper phthalocyanine. The second preferred compound can be obtained by replacing the potassium perchlorate with ammonium perchlorate and potassium nitrate in ratios which provide the same ion concentration of K+, ClO4xe2x88x92 in the remaining mixture of AN/GN and/or AGN/PVA.
The burning rate of the gas generating composition is increased and the pressure exponent of the gas generating composition is simultaneously lowered without adversely affecting other propellant properties by the addition of an effective amount of copper or a selected copper compound to a solid solution or eutectic mixture of the components of the gas generating composition, which is adjusted to maintain the oxygen to fuel ratio in the desired range of 0.88 to 1.0. This entails an increase in AN and decrease in GN and/or AGN.
Additional objects and advantages of the present invention will be apparent to those skilled in this art from the following description, claims and drawings. The scope of the present invention is capable of other and different embodiments and modifications, and the description and drawings are intended to be illustrative, not restrictive.